Reactions in Aqueous Solutions Chapter 7

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Reactions in Aqueous SolutionsChapter 7
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Predicting Whether a Reaction Will Occur

• Forces that drive a reaction
• formation of a solid
• formation of water
• transfer of electrons
• formation of a gas
• when chemicals (dissolved in water) are mixed and
  one of these 4 things can occur, the reaction
  will generally happen

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Precipitation Reactions

• in all precipitation reactions, the ions of one
  substance are exchanged with the ions of another
  substance when their aqueous solutions are mixed
• At least one of the products formed is insoluble
  in water
• KI(aq) AgNO3(aq) ? KNO3(aq) AgI?s?

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Figure 7.1 The precipitation reaction that
occurs when yellow potassium chromate, K2CrO4
(aq), is mixed with a colorless barium nitrate
solution, Ba(NO3)2 (aq)
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Dissociation

• ionic compounds
• metal nonmetal (Type I II)
• metal polyatomic anion
• polyatomic cation anion
• when ionic compounds dissolve in water the anions
  and cations are separated from each other this
  is called dissociation
• we know that ionic compounds dissociate when they
  dissolve in water because the solution conducts
  electricity

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Figure 7.2 Electrical conductivity of aqueous
solutions
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Dissociation

• potassium chloride dissociates in water into
  potassium cations and chloride anions
• KCl(aq) K (aq) Cl- (aq)
• copper(II) sulfate dissociates in water into
  copper(II) cations and sulfate anions
• CuSO4(aq) Cu2(aq) SO42-(aq)

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Dissociation

• potassium sulfate dissociates in water into
  potassium cations and sulfate anions
• K2SO4(aq) 2 K (aq) SO42-(aq)

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Predicting the Products ofa Precipitation
Reaction

• 1. Determine what ions each aqueous reactant has
• 2. Exchange Ions
• () ion from one reactant with (-) ion from other
• 3. Balance Charges of combined ions to get
  formula of each product
• 4. Balance the Equation
• count atoms
• 5. Determine Solubility of Each Product in Water
• solubility rules
• if product is insoluble or slightly soluble, it
  will precipitate

Pb(NO3)2 (aq) CaCl2 (aq) ----gt
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Solubility Rules

• 1. Most compounds that contain NO3- ions are
  soluble
• 2. Most compounds that contain Na, K, or NH4
  ions are soluble
• 3. Most compounds that contain Cl- ions are
  soluble, except AgCl, PbCl2, and Hg2Cl2
• 4. Most compounds that contain SO42- ions are
  soluble, except BaSO4, PbSO4, CaSO4
• 5. Most compounds that contain OH- ions are
  slightly soluble (will precipitate), except NaOH,
  KOH, are soluble and Ba(OH)2, Ca(OH)2 are
  moderately soluble
• 6. Most compounds that contain S2-, CO32-, or
  PO43- ions are slightly soluble (will
  precipitate)

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Figure 7.3 Solubilities of common compounds
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Figure 7.4 Precipitation of silver chloride
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Ionic Equations

• equations which describe the chemicals put into
  the water and the product molecules are called
  molecular equations
• KCl(aq) AgNO3(aq) ? KNO3(aq) AgCl(s)
• equations which describe the actual ions and
  molecules in the solutions as well as the
  molecules of solid, liquid and gas not dissolved
  are called ionic equations
• K (aq) Cl- (aq) Ag (aq) NO3- (aq) ??K
  (aq) NO3- (aq) AgCl(s)

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Ionic Equations

• ions that are both reactants and products are
  called spectator ions
• K (aq) Cl- (aq) Ag (aq) NO3- (aq) ??
  K (aq) NO3- (aq)
• AgCl(s)
• an ionic equation in which the spectator ions
  are dropped is called a net ionic equation
• Cl- (aq) Ag (aq) ??AgCl(s)

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Electrolytes

• electrolytes are substances whose aqueous
  solution is a conductor of electricity
• all electrolytes have ions dissolved in water
• in strong electrolytes, virtually all the
  molecules are dissociated into ions
• in nonelectrolytes, none of the molecules are
  dissociated into ions
• in weak electrolytes, a small percentage of the
  molecules are dissociated into ions

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Figure 7.5 When gaseous HCl is dissolved in
water, each molecule dissociates to produce H
and Cl- ions
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Reactions that Form WaterAcids Bases

• Acids all contain H cations and an anion
• HCl, HBr, HI, HNO3, H2SO4
• Bases all contain OH- anions and a cation
• NaOH, KOH, NH4OH
• when acids dissociate in water they release H
  ions and their anions
• when bases dissociate in water they release OH-
  ions and their cations

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Acid-Base Reactions

• in the reaction of an acid with a base, the H1
  from the acid combines with the OH-1 from the
  base to make water
• the cation from the base combines with the anion
  from the acid to make the salt
• acid base ???salt water
• H2SO4(aq) Ca(OH)2(aq) ? CaSO4(aq) 2 H2O(l)
• the net ionic equation for an Acid-Base reaction
  is always
• 2 H (aq) 2 OH- (aq) ? 2H2O(l)

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Reactions of Metals with Nonmetals(Oxidation-Redu
ction)

• The metal loses electrons and becomes a cation
• We call this process oxidation
• The nonmetal gains electrons and becomes an anion
• We call this process reduction
• In the reaction, electrons are transferred from
  the metal to the nonmetal

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Oxidation-Reduction Reactions

• All reactions that involve a transfer of one or
  more electrons are called oxidation-reduction
  reactions
• We say that the substance that loses electrons in
  the reaction is oxidized and the substance that
  gains electrons in the reaction is reduced.

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Predicting Products of Metal Nonmetal Reactions

• metal nonmetal ? ionic compound
• ionic compounds always solids unless dissolved in
  water
• in the ionic compound the metal is now a cation
• in the ionic compound the nonmetal is now an
  anion
• to predict direct synthesis of metal nonmetal
• determine the charges on the cation and anion
• from their position on the Periodic Table
• determine numbers of cations and anions needed to
  have charges cancel
• balance the equation

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Oxidation-Reduction Examples

• 2 Na (s) Cl2 (g) ----gt 2 NaCl (s)
• 2 Mg (s) O2 (g) ----gt 2 MgO (s)
• 2 Al (s) Fe2O3 (s) ----gt 2 Fe (s) Al2O3
  (s)
• 2 Al (s) 3 I2 (s) ----gt 2 AlI3 (s)

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Another Kind of Oxidation-Reduction Reaction

• Some reactions between two non-metals are also
  oxidation-reduction reaction
• Any reaction in which O2 is a reactant or a
  product will be an oxidation-reduction reaction
• CH4(g) 2 O2(g) ? CO2(g) 2 H2O(g)
• 2 SO3(g) ? 2 SO2(g) O2(g)

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Figure 7.8 For launch, the space shuttle orbiter
is attached to two solid-fuel rockets and a fuel
tank that supplies hydrogen and oxygen to the
orbiters engines
2 H2 (g) O2 (g) ----gt 2 H2O (g) ENERGY
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Ways to Classify Reactions

• Reactions that involve solid formation are called
  precipitation reactions
• Reactions that involve water formation are called
  acid-base reactions
• Both precipitation reactions and acid-base
  reactions involve compounds exchanging ions, ion
  exchange reactions are called double displacement
  reactions

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Double Displacement Reactions

• two ionic compounds exchange ions
• X ?Y? (aq) A??B? (aq) ? XB AY
• reaction will not occur unless one of the
  products either (1) precipitates, (2) or is water

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Ways to Classify Reactions

• Reactions that involve electron transfer are
  called oxidation-reduction reactions
• Metals Nonmetal
• O2 as a reactant or product
• Reactions that occur in aqueous solution because
  one of the products is a gas are called gas
  forming reactions
• NaHCO3(aq) HCl(aq) ? NaCl(aq) CO2(g) H2O(l)

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Ways to Classify Reactions

• Reactions that involve one ion being transferred
  from one cation to another are called single
  replacement reaction
• X ?Y? A??? X A?Y?
• Zn(s) 2 HCl(aq) ? ZnCl2(aq) H2(g)
• Fe2O3(s) 2 Al(s) ? 2 Fe(s) Al2O3(s)

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Other Ways to Classify Reactions

• Reactions in which O2(g) is reacted with a carbon
  compound are called Combustion Reactions
• Combustion reactions release a lot of energy
• Combustion reactions are a subclass of
  Oxidation-Reduction reactions
• Combustion of carbon compounds produces CO2(g)
• Combustion of compounds that contain hydrogen
  produces H2O(g)
• C3H8(g) 5 O2(g) ? 3 CO2(g) 4 H2O(g)

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Other Ways to Classify Reactions

• Reactions in which chemicals combine to make one
  product are called Synthesis Reactions
• Metal Nonmetal reactions can be classified as
  Synthesis Reactions
• 2 Na(s) Cl2(g) ? 2 NaCl(s)
• Reactions of Metals or Nonmetals with O2 can be
  classified as Synthesis Reactions
• N2(g) O2(g) ? 2 NO(g)
• These two types of Synthesis Reactions are also
  subclasses of Oxidation-Reduction Reactions

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Other Ways to Classify Reactions

• Reactions in which one reactant breaks down into
  smaller molecules are called Decomposition
  Reactions
• Generally initiated by addition of energy
• Addition of electric current or heat
• Opposite of a Synthesis Reaction
• 2 NaCl(l) ? 2 Na(l) Cl2(g)

electric current
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Figure 7.11 Classes of reactions
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Figure 7.12 Summary of classes of reactions